In recent years, there has been known a digital photocopier product which electronically scans a document image with a scanner device, and outputs the record of the image data thus supplied from the scanner device via a printer device. Such a digital photocopier has been lately developed into a digital multi-functional device which is capable of dealing with a network with a communication function (e.g. facsimile function, scanner function, printer function) for carrying out sending/receiving of image data to/from an external device. Further, such multi-function of the digital photocopier has been improved to be able to effectively process a large amount of image data and a plurality of jobs, by having a built-in storage section for temporarily storing image data.
Meanwhile, there are various types of storage device for storing image data, for example, one including a semiconductor memory or a hard disk device made up of a magnetic storage medium. These storage devices have different environments for storing image data, for example, they are differ in storage capacitance, costs, writing speed, reading speed, or volatility/non-volatility.
Particularly, in a magnetic storage medium such as a hard disk device, a disk (storage medium) is divided into small units by using a management data called FAT data so as to carry out control of each area of the units. Then, the FAT data is renewed when data is stored in the respective areas in the disk (storage medium), and the data stored in the respective areas are appropriately read out based on the FAT data.
Presently, because of its desirable storage capacitance, costs (cost of components), and data transfer speed, this type of hard disk device is mounted in a digital multi-functional device as an auxiliary storage device of a semiconductor memory. The hard disk device processes a certain amount of job data on the semiconductor memory, while temporarily storing newly supplied job data, and reading out the stored job data in turn onto the semiconductor memory for carrying out processing of the data. This processing system has been appearing on the market as an efficient image processing system (digital multi-functional device) for dealing with a large amount of job data without causing holdup of the data.
However, such a digital multi-functional device is disadvantageous in terms of security when handling a confidential document data. For example, the hard disk device persistently keeps the job data (image data) stored in the respective areas of the hard disk device even after processing, until the digital multi-functional device judges that the job data is no longer required and renews the FAT data so that newly supplied job data is written on the area which has been keeping the job data no longer required, or unless the job data no longer required is compulsively deleted from the area.
One example aimed at overcoming this problem can be found in Japanese Laid-Open Patent Application Tokukaihei 09-284572/1997 (published on Oct. 31, 1997, hereinafter referred to as a patent document 1), which discloses a digital multi-functional device with a function for preventing reproduction of the processed image, by compulsively deleting the area keeping the image data in the hard disk device, or by writing an irregular pattern generated by random numbers onto the area, when the processing of the data is completed.
However, according to the patent document 1, the digital multi-functional device takes a while to complete the foregoing measures for invalidating the image data by compulsively deleting the area keeping the image data, or by writing an irregular pattern generated by random numbers onto the area. Particularly, the processing for invalidation with respect to all image data storage areas throughout the hard disk device takes a long time to complete, and holds the user up to carry out other functions of the digital multi-functional device.